Human chorionic gonadotropin is secreted during the early stages of pregnancy, the book said, and helps maintain the corpus luteum. The corpus luteum secretes progestogens and estrogen to preserve the inner lining of the womb and prevent menstruation. In this way the placenta gradually takes form. In seven to nine weeks, once the placenta is complete, there is no more need for the corpus luteum and the role of the human chorionic gonadotropin is over.